(1) Field of the Invention
The present invention relates to composite compositions which comprise wood particles and polyvinyl chloride (PVC) polymers wherein chitin and/or chitosan are used as natural coupling agents. The resulting composite compositions have improved properties.
(2) Description of the Related Art
Wood-plastic composites (WPCs) have emerged as an important family of engineering materials. They are partially replacing solid pressure-treated wood and other materials in a variety of applications (Clemons, C. M., Forest Prod. J., 52(6), 10-18 (2002). Although WPCs are superior to the unfilled neat polymers in terms of material cost and stiffness, their strength performance (tensile, flexural, and impact) is generally lower than the unfilled polymers (Matuana, L. M., Woodhams, R. T., Balatinecz, J. J. and Park, C. B., Polym. Compos., 19(4), 446-455 (1998); and Matuana, L. M., Balatinecz, J. J., and Park, C. B., Polym. Eng. Sci., 38(5): 765-773 (1998)). The decreased strength is likely a result of the natural incompatibility of phases during the mixing of the hydrophilic wood fibers (high surface tension) with the hydrophobic polymer matrix (low surface tension) (Matuana, L. M., Woodhams, R. T., Balatinecz, J. J. and park, C. B., Polym. Compos., 19(4), 446-455 (1998); Matuana, L. M., Balatinecz, J. J., and Park, C. B., Polym. Eng. Sci., 38(5): 765-773 (1998); Li, W. and Matuana, L. M., J. Appl. Polym. Sci., 88(2), 278-286 (2003); and Matuana, L. M., Park, C. B. and Balatinecz, J. J., J. Vinyl Addit. Technol., 3(4), 265-273 (1997)). Phase incompatibility yields very weak interactions and thus a weak interface (poor interfacial adhesion) between the fiber and the matrix.
One approach to designing WPCs is to modify the wood fiber surface with coupling agents to improve the strength (Woodhams, R. T., Thomas, G. and Rodgers, D. K., Polym. Eng. Sci., 24: 1166-1177 (1984). Coupling agents convert the hydrophilic surface of wood fibers to a more hydrophobic one, thereby reducing the surface tension of wood fibers so that they more closely match that of the molten polymer. As a result, wetting and adhesion are improved via mechanisms such as diffusion and mechanical interlocking between treated fibers and the polymer matrix (Woodhams, R. T., Thomas, G. and Rodgers, D. K., Polym. Eng. Sci., 24: 1166-1177 (1984).
Due to its strong effect in altering the hydrophilic surface of wood fibers to a more hydrophobic one, maleic anhydride functionalized polyolefin is commonly used as an appropriate coupling agent for polyolefin/wood-fiber composites (Li, W. and Matuana, L. M., J. Appl. Polym. Sci., 88(2), 278-286 (2003); and Woodhams, R. T., Thomas, G. and Rodgers, D. K., Polym. Eng. Sci., 24: 1166-1177 (1984)). Similarly, to enhance the interfacial adhesion between wood fibers and PVC matrix, the second largest plastic used to manufacture WPCs, several investigators have assessed the effects of various fiber treatments, including different types of isocyanates, maleic anhydride, silanes, etc. as coupling agents (Kokta, B. V., Maldas, D., Daneault, C. and Beland, R., J. Vinyl Technol., 12(3), 146-153 (1990); Kokta, B. V., Maldas, D., Daneault, C. and Beland, P., Polum. Plast. Technol. Eng., 29(1-2), 87-118 (1990); and Kokta, B. V., Maldas, D., Daneault, C. and Beland, R., Polym. Compos., 11(2), 84-89 (1990)). Most mechanical properties of the composites were improved by these chemical treatments compared to those of composites with non-treated fibers. However, the properties of the composites were inferior to those of the unfilled PVC, suggesting that, unlike polyolefin/wood-fiber composites, the well-known claim of converting the hydrophilic surface of wood-fiber to hydrophobic one is not effective for enhancing the adhesion of PVC to wood-fibers.
Our previous studies, however, demonstrated that when PVC is used as matrix in WPCs, acid-base interactions, in which one phase acts as an electron donor (base) and the other acts as an electron acceptor (acid), are a significant factor in enhancing interfacial adhesion (Matuant, L. M., Woodhams, R. T., Balatinecz, J. J. and Park, C. B., Polym. Compos., 19(4), 446-455 (1998); and Matuana, L. M., Balatinecz, J. J., and Park, C. B., Polym. Eng. Sci., 38(5): 765-773 (1998)). Therefore, surface modification of wood fibers to be used with PVC should be designed to modify the acid-base interactions at the matrix/fiber interface in order to improve the performance of these composites. For Example, by changing the acidic characteristics of wood fibers through surface modification with γ-aminopropyltriethoxy silane, PVC/wood-fiber composite with equal tensile strength and greater modulus than unfilled PVC was developed (Matuana, L. M., Woodhams, R. T., Balatinecz, J. J. and Park, C. B., Polym. Compos., 19(4), 446-455 (1998). The use of the aminosilane successfully modified the wood surface, and facilitated the interaction between the wood and PVC according to Lewis acid-base theory (Matuana, L. M., Woodhams, R. T., Balatinecz, J. J. and Park, C. B., Polym. Compos., 19(4), 446-455 (1998).
In spite of these benefits, γ-amino propyltriethoxy silane has not been extensively used as a coupling agent for PVC/wood-fiber composites, mainly due to its high cost but also due to the difficulty in evenly coating the surface of wood fibers, owing to its sensitivity to hydrolyze and self-condense. Consequently, aminosilane is not a desirable coupling agent in this application.
Chitin (FIG. 1A) is the second most abundant natural polymer after cellulose and is extracted from the shells of crustaceans. Chitosan (FIG. 1B) is the deacetylated form of chitin. These polymers are widely available, non-toxic, biocompatible, and lower in cost than many synthetic coupling agents. The acetyl amine functionality of chitin, and the amine functionality of the chitosan, should permit these polymers to interact with wood and PVC in a manner similar to the aminosilane, and so enhance the interfacial adhesion of between PVC and wood fibers, while potentially improving other properties, and also be more cost-effective.
Several investigators have reported the use of chitin and chitosan with different polymers for various applications (Yang, A. and Wu, R., J. Appl. Polym. Sci., 84(3), 486-492 (2002); Zhang, W., Liu, L., Ren, L. and Wang, F., J. Appl. Polym. Sci., 64(11), 2127-2130 (1997); Souza Rosa, R. C. R. and Andrade, C. T., J. Appl. Polym. Sci., 92(4), 2706-2713 (2004); Ratajska, R. and Boryniec, S., Polym. Adv. Technol., 10(10), 625-633 (1999); Thwe, M. M. and Liao, K., Plast. Rubber Compos., 31(10), 422-431 (2002); Sato, K., Ota, H. and Omura, Y., Adv. Chitin Sci., 2, 897-901 (1997) and Umemura, K., Inoue, A. and Kawai, S., J. Wood Sci., 49(3), 221-226 (2003)). In one study, the interfacial adhesion between chitin fibers and polycaprolactone (PCL) was increased by an irradiation treatment of the composites. The treatment showed an overall increase in mechanical properties of the composite compared to composite prepared from untreated chitin fiber. This increase in interfacial bonding was attributed to a free-radical grafting reaction (Yang, A. and Wu, R., Appl. Polym. Sci., 84(3), 486-492 (2002). Chitosan has also been reported to have been cross-linked to a polymer matrix (Zhang, Q., Liu, K., Ren, L. and Wang, F., J. Appl. Polym. Sci., 64(11), 2127-2130 (1997). That process uses formaldehyde as a cross-linking agent which is a known carcinogen and hazardous to the environment.
Biodegradable composites were also prepared by incorporating chitin flakes ranging from 0-30 wt % into a plasticized starch matrix by an injection molding process. Chitin flakes increased the elastic modulus, tensile stress and water resistance of the composites when compared to the unfilled starch (Souza Rosa, R. C. R. and Andrade, C. T., J. Appl. Polym. Sci., 92(4), 2706-2713 (2004). Biodegradation of the synthetic polymers can also be increased by incorporating a natural biodegradable polymer such as chitosan (Ratajska, M. and Boryniec, S., Polym. Adv. Technol., 10(10), 625-633 (1999).
Chitosan is also used in the wood industry. For Example, chitosan forms a Schiff base when reacted with aldehyde compounds. This property of chitosan can be very useful in reacting with the formaldehyde released from the glue line of plywood, thus reducing the overall emission of formaldehyde to the environment. Hence, the chitosan can be used as a functional coating reagent for wood (Sato, K., Ota, H. and Omura, Y., Adv. Chitin Sci., 2, 897-901 (1997). The use of chitosan as an environmentally friendly adhesive for wood has also been reported in the literature. Glue made from chitosan showed excellent water resistance and was proposed as a replacement for synthetic adhesives (Umemura, K., Inoue, A. and Kawai, S., J. Wood Sci., 49(3), 221-226 (2003).
The prior art has not disclosed the use of chitin or chitosan as adhesion promoters in PVC/wood-fiber composites.